1. Field of the Invention
The present invention relates to a light-emitting element. Specifically, the present invention relates to a light-emitting element using organic electroluminescence (EL). The present invention also relates to a light-emitting device, an electronic device, and a lighting device each including the light-emitting element.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. In a basic structure of such a light-emitting element, a light-emitting layer containing a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
Such a light-emitting element is of self-luminous type, and thus has advantages over a liquid crystal display in that visibility of pixels is high, a backlight is not needed, and so on. Therefore, such a light-emitting element is regarded as being suitable as a flat panel display element. Besides, such a light-emitting element has advantages in that it can be manufactured to be thin and lightweight, and has very fast response speed.
Since such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission. Thus, a large-area element utilizing planar light emission can be easily formed. This is a feature that is difficult to obtain with point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Therefore, the light-emitting element is very effective for use as a surface light source applicable to lighting and the like.
Such light-emitting elements utilizing electroluminescence can be broadly classified according to whether a light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as a light-emitting substance is provided between a pair of electrodes, application of voltage to the light-emitting element causes injection of electrons from a cathode and holes from an anode into the layer containing the organic compound having a light-emitting property and thus current flows. The injected electrons and holes then lead the organic compound having a light-emitting property to its excited state, whereby light emission is obtained from the excited organic compound having a light-emitting property.
Note that excited states of the organic compound include a singlet excited state and a triplet excited state. Light emission from the singlet excited state (S*) is called fluorescence, and light emission from the triplet excited state (T*) is called phosphorescence. The statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3.
At room temperature, a compound capable of converting a singlet excited state into luminescence (hereinafter, referred to as a fluorescent compound) exhibits only luminescence from the singlet excited state (fluorescence), and luminescence from the triplet excited state (phosphorescence) cannot be observed. Accordingly, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
In contrast, a compound capable of converting a triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound) exhibits luminescence from the triplet excited state (phosphorescence). Further, since intersystem crossing (i.e., transition from a singlet excited state to a triplet excited state) easily occurs in a phosphorescent compound, the internal quantum efficiency can be theoretically increased to 100%. That is, higher emission efficiency can be achieved than using a fluorescent compound. For this reason, light-emitting elements using a phosphorescent compound have been under active development recently so that high-efficiency light-emitting elements can be achieved.
When a light-emitting layer of a light-emitting element is formed using the phosphorescent compound described above, in order to suppress concentration quenching or quenching due to triplet-triplet annihilation of the phosphorescent compound, the light-emitting layer is usually formed such that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound serving as the matrix is called host material, and the compound dispersed in the matrix like the phosphorescent compound is called guest material.
When the phosphorescent compound is used as the guest material, the host material is required to have a higher triplet excitation energy level (difference in energy between the ground state and the triplet excited state, which is also referred to as T1 level) than the phosphorescent compound.
Since the singlet excitation energy level (difference in energy between the ground state and the singlet excited state, which is also referred to as S1 level) is higher than a T1 level, a substance that has a high T1 level also has a high S1 level. Therefore, the above substance that has a high T1 level is also effective in a light-emitting element using a fluorescent compound as a light-emitting substance.
Studies have been conducted on compounds having a dibenzo[f,h]quinoxaline skeleton, which are examples of the host material used when a phosphorescent compound is a guest material (e.g., see Patent Documents 1 and 2).